There are many applications in industry for wire tensioning devices. In security systems, for instance, a set of parallel wires may be stretched around the perimeter of an area; the wires acting as capacitors. When the dielectric constant between the wires changes because an intruder or other object has changed the characteristic of the dielectric, the capacitance changes and triggers an alarm. In this type of system it is imperative that the wires are parallel, that they retain a relatively constant tension and that they are insulated from any support poles. These requirements are not trivial because variations of temperature can cause the wire to expand and contract. The change in length of the wire is a function of the original length of the wire, the change in temperature, and the coefficient of expansion of the material from which the wire is made .
Previous attempts to solve the difficulties associated with keeping tension on wires have not been entirely successful. One way to compensate for the changes of length due to thermal expansion is to attach helical springs to the ends of the wires, the helical springs expanding when the temperature drops and the wires contract, and contracting when the associated wire expands. The use of helical springs, however, has several prohibitive drawbacks. First, the helical spring itself does not act as a capacitor and therefore there is a dead spot in the security system which extends through the spring, through the pole to which the spring is attached, and possibly through a spring on the other side of the pole if the security fence continues. This dead zone increases as the ambient temperature decreases because the springs expand.
The second problem associated with helical springs is that the springs cannot be attached directly to support poles unless either the poles or the springs are electrically insulating material. Typically, the fence posts utilized in industry are metallic and replacing these posts would be costly. One way that industry has dealt with the problem of attaching metallic helical springs to a metallic post is by attaching a non-metallic bracket to the post and then connecting the spring to the bracket. This procedure has high installation and material cost, however.